Stabilization of charged particle beams



APril 14, 1970 R. J. BRIGGS 3,506,865

Filed July 28, 1967 INVENTOR. RICHARD J. BR! 66 S ATTORNEY United States Patent Oflice 3,506,865 Patented Apr. 14, 1970 3,506,865 STABILIZATION F CHARGED PARTICLE BEAMS Richard J. Briggs, Lexington, Mass, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed July 28, 1967, Ser. No. 656,956 Int. Cl. H0511 11/00, 13/04 US. Cl. 313--62 Claims ABSTRACT OF THE DISCLOSURE Charged particle beam conductive or directing apparatus having an electrically inductive structure disposed circumjacent said beam to eliminate the tendency for beam particles to bunch by the interaction of electric charges induced in the inductive structure within the beam.

BACKGROUND OF THE INVENTION This invention was made in the course of, or under. Contract W7405-ENG-48 with the United States Atomic Energy Commission.

FIELD OF THE INVENTION This invention relates generally to the stabilization of a charged particle beam by providing for maintenance of uniform density distribution of charged particles in a beam, and more particularly, it pertains to apparatus for maintaining a uniform charge distribution along the path of charged particle beams to minimize negative mass instability disruption of said beam.

DESCRIPTION OF THE PRIOR ART A fundamental limitation of the intensity of beams used in accelerators, storage rings, etc., has been the negative mass instability. This phenomenon is described by Nielsen, Sessler and Symon in the article Longitudinal Instabilities in Intense Relativistic Beams appearing in the International Conference on High-Energy Accelerators and Instrumentation-Cern Proceedings, 1959, page 239. Briefly, the negative mass instability disrupts the uniformity of the charge distribution, hereinafter referred to simply as the density of charged particle beam traveling in curved paths by causing the bunching of charged particles or producing localized density perturbations when the beam is accelerated to high velocities. Said instability arises from the fact that a force acting on an ion in the direction of motion of an ion so as to increase its energy concomitantly decreases its frequency of revolution, i.e., the angular acceleration is in a direction opposite to the direction of applied torque. This behavior is just the opposite to what intuitive reasoning might predict, hence the name negative mass instability. One prior art apparatus for eliminating negative mass instability of charged particle beams was described in my prior US. Patent No. 3,324,325, issued June 6, 1967, and entitled Dielectric Wall Stabilization of Intense Charged Particle Beams wherein a dielectric coating on the walls of charged particle beam guide interacts with the beam to provide stabilization. Another method proposed for eliminating negative mass instability involves the use of an energy spread among beam particles, which method is complicated and inherently eliminates the characterstic property of a mono-energetic beam as required in many research applications.

SUMMARY An object of this invention is to provide a simple structure, e.g., easily installable and removable apparatus, for stabilizing a beam against density perturbations, namely charged particle bunching, such as that which produces negative mass instability disruption of intense charged particle beams. Apparatus which achieves this object includes an inductive conductor structure disposed circumjacent said beam, e.g., a toroidal helix for impeding the flow of charges induced in said structure by the beam. The charges induced in the helix by the ions in the beam experience increased inertia in forming the deleterious particle bunches in the beam. Hence, particles in the beam tend to remain at uniformly distributed positions or to be restored to a uniform distribution of said inertia effect. The beam density distribution is thus stabilized against density perturbations and resultant negative mass instability of the beam.

BRIEF DESCRIPTION OF THE DRAWINGS Objects and features of the invention will be apparent in the following description and accompanying drawings, of which:

FIGURE 1 is a plan view of a preferred embodiment of the invention, illustrating an inductive negative mass stabilization structure installed in a curved charged particle beam guide;

FIGURE 2 is a view of the inductive structure of FIG- URE 1;

FIGURE 3 is a plan view of a second preferred embodiment of the inductive structure of the invention; and

FIGURE 4 is a perspective view of another inductive stabilization structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Generally speaking, inductive stabilization in accord with the invention may be applied in any device in which a charged particle beam, confined by a magnetic field or otherwise, is directed along an elongated, particularly, curved or endless path. Such devices may include charged particle accelerators, certain controlled fusion reactors, storage ring apparatus, toroidal circulating beam devices, etc.

More particularly with reference to FIGURE 1, there is shown a toroidal circulating particle beam apparatus in which a magnetic field l1 generated by magnets (not shown) guides a charged particle beam 12, e.g., charged positive particle beam of deuterons, protons, tritons, etc., in a circular orbit along the axial path of a magnetically permeable toroidal beam guide housing 13. Beam 12 may be accelerated within housing 13 to a specified velocity by any of the means well known in accelerator physics, e.g., betatron, toroidal pinch, or by injection of energetic particles. At elevated particle velocities, particle bunching occurs. The mechanism which initiates bunching and drives the bunches to ever-increasing size is described in my prior patent mentioned above. Although the mechanism referred to is the negative mass instability, other instabilities could also cause density perturbation or unwanted density modulation of beam 12. Briefly, as illustrated in FIGURE 5, the charged particles of the beam 12 may gather in bunches 16 at spaced intervals along a curved path 17 traversed by beam 12, as caused by the mechanism discussed above. Hence, beam 12 is defined by particle bunches 16 spaced apart by void spaces 18 which may be represented by positive and negative signs, respectively. Although the beam in FIGURE 5 is shown to be comprised of ions, my invention would apply to electron beams, in which case the charge signs in the drawing would be reversed.

Considering now the particle bunch 16 in a conventional curved beam guide as provided in apparatus described above, the positive particles at the head 19 of bunch 16 experience a forward longitudinal force, i.e., in the direction of arrow 17. Moreover, the positive particles at the tail 20 of bunch 16 experience a backward longitudinal force. Where the particles orbiting frequency is a decreasing function of energy, for example, above the transition energy in strong focusing charged particle accelerators, the longitudinal forces increase the Orbiting radius and decrease the angular velocity of the particles at the head 19 of beam 12 so that such particles move backward toward the region of increased density of bunch l6. Simultaneously, the longitudinal force experienced by the particles at the tail 20 of bunch 16 decreases the orbiting radius and increases the angular velocity of the particles, and the particles move forward toward the region of increased density of bunch 16. As the bunch 16 becomes denser, the tendency of the particles to bunch becomes greater. Consequently, in magnetically directed charged particle accelerators and beam guides having a curved beam path segment, beam intensity limitations are encountered at particle beam energies where the particle orbiting frequency decreases with an increase in energy.

I have found that if segments of the beam guide structure which proximate the curved beam path presents an effective longitudinal inductive impedance to a charged particle beam magnetically directed therethrough, the conditions giving rise to the negative mass instability will be eliminated; hence the concomitant instability induced particle bunching or uniform particle distribution in the beam is minimized or eliminated.

A toroidal helix 21 unit or structure provided as shown in FIGURE 2 will eliminate the negative mass instability. It may be mounted inside a beam guide as shown in FIG- URE 1 or outside a nonconductive beam guide as illustrated in FIGURE 3. Toroidal helix unit 21 is provided with successive conductor turns whose planes are generally perpendicular to the beam 12 when installed as shown in FIGURE 1. Conductive material 22 from which the turns are made preferably is a thin ribbon-like or flattened material of a form similar to tape. A tape-type material will allow termination of electric field lines (shown as short arrows in FIGURE over a wide area. Field lines converging on a large area will not be appreciably bent, and electric field inhomogeneities will be avoided. Tape foils made from copper, silver, aluminum, etc., are suitable.

Many accelerators employ nonconductive magnetically permeable beam guides such as glass. In this case, conductive material 22 can be applied directly to or wound along the beam guide 13 and held in place, e.g., by nonconducting adhesive. The toroidal helix 21 as illustrated in FIGURE 1 is adhesively fastened to beam guide 13. Each turn 22 of the toroidal helix 21 is provided with inductance and resistance properties of appropriate magnitude to reduce the charged particle bunching effect as described more fully below.

In operation, charged particles bunch as charged particle beam 12 is accelerated to a high velocity, i.e., those velocities above about 0.1 times the speed of light. In FIGURE 5, it will be seen that the positively charged particles of bunch 16 induce negative charges in the regions 32 radially surrounding the beam 12, as well as in the turns of helix 21. Similarly, the negative void region 18 induces positive charges in the regions 33 radially surrounding the beam 12. It can readily be seen that, as the ions of bunch 16 move in the direction of the arrow 17, the charges 32 induced in the helix travel a greater distance than the beam particles, due to the longer path length along helix 21, in order to maintain the same relative longitudinal spacing with respect to the particle bunches which induce such charges in the beam. At high beam velocities (above about 0.1 times the speed of light), the induced charges tend to fall behind or be retarded by the increased path length since the induced charges must spiral around the beam with a velocity which is greater than the velocity of the beam by an amount equal to ZTrRor, where R is the radius of a turn, and a is the helical pitch of the turn. The inertia of such retardation effect produced on the movement of the induced charges 15 communicated by interaction with beam particle charges to prevent or oppose changes productive of non-uniformity in the beam particle distribution. Hence, the density distribution of a beam entering or traveling along the axial portion of helix 21 will be stabilized.

In constructing a helix as shown in FIGURE 2, I have found that the choice of an appropriate number and spacing or pitch of the turns is governed by the inequality v LC l, where v is the velocity of particles in the beam, L is the inductance per unit length of the coil which is proportional to the number and spacing of turns according to well known formulas, and C is the capacitance between the beam and the wall surrounding it before the inductor is affixed thereto.

In making an inductor, there will be some stray capacitance between turns of the inductor. Estimates of this stray capacitance should be included in calculating or measuring C In the embodiment of FIGURE 4, a charged particle beam 12 travels through spaced conductive wedge-shaped sections 23 of a beam guide of the type employed in a variety of particle accelerators. Successive wedge-shaped sections 23, having side edge portions parallel and defining a beam passageway, are disposed circumjacent beam 12. In accord with the invention, a helical inductor, 24, 25, 26, 27, et seq., is electrically connected in series between adjacent ends of said sections 23. Once again the electrical value of the inductor is given by the formula v LC l explained above. Wedge-shaped sections 23 with coils 24, 25, 26, et seq., are generally housed in a vacuum housing (not shown) wherein a vacuum environment may be established.

Example A beam of charged particles may be stabilized with apparatus as shown in FIGURE 3, having the following dimensions:

Beam energy300 kev.

Particle velocity ratio v /c 6.6 10

Beam 12 (diameter)1 cm.

Orbit radius (R)-8 cm.

Magnetic field strengthl0 kilogauss Frequency of revolution-44.6 mc./sec.

Radial separation of helix from beam-2 cm.

Coil inductance-HIP henries per meter Beam to wall capacitance-1.7x 10- farads per meter using the structure shown in FIGURE 2 in existing apparatus.

While the description of the invention and the above example relate to specific preferred embodiments, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention. Accordingly, the only limits intended for my invention are those defined in the following claims.

I claim:

1. In apparatus of a class including means for producing a magnetic field in an evacuated region and means for generating a beam of charged particles traveling along a substantially circular orbital path in said magnetic field, said particles having a velocity of above about (1.1 times the speed of light whereat particle bunching or perturbations of the particle density can occur to produce charges of opposite polarity in the vicinity of the particle bunches and intervening spaces respectively, therebetween, the combination therewith of electrical conductor means including segments disposed sequentially circumjacent said charged particle beam in which segments electrical charges of a polarity correspondingly opposite to that of said particle bunches and intervening spaces are induced, said segments being serially connected by means of inductive conductor elements providing a closed path about the total length of said circular orbital beam path, said inductive elements having an inductance of sufficient magnitude cooperating with the capacitance of the system to impede the flow of said induced charges therealong to thereby impede the formation of said charged particle bunches and particle density perturbations in the charged particle beam.

2. Apparatus as defined in claim 1 wherein said electrical conductor means is a tape or ribbon-like conductor wound in the form of a toroidal helix with the wide dimension thereof facing and encompassing said particle beam orbital path with the individual turns thereof being spaced from each other and defining said segments disposed circurnjacent said charged particle beam and wherein said inductive conductor elements comprise the distributed inductances of the turns of said helix.

3. Apparatus as defined in claim 2 which further includes a magnetically permeable toroidal beam guide housing enclosing said circular orbital beam path and said tape or ribbon-like helical conductor means is disposed and supported coaxially with respect to said beam guide housmg.

4. Apparatus of claim 3, further defined in that the inductance of said inductors satisfies the inequality:

References Cited UNITED STATES PATENTS 2,297,305 9/1942 Kerst 3l362 X 2,569,154 9/1951 Donath 3153l 2,822,491 2/l958 Wideroe 3l362 2,825,833 3/1958 Yanagisawa et a]. 3l362 2,890,348 6/1959 Ohkawa 328235 X 3,324,325 6/1967 Briggs 3l362 JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R. 3l55; 328233 

